1. Field of the Invention
The present invention relates generally to control of orbiting spacecraft, especially for controlling spacecraft yaw and roll excursions caused by solar torques and thruster firings and, more particularly, for augmenting magnetic torquer management of solar torque with thruster availability in case larger than expected external torques are imparted on the spacecraft.
2. Description of the Prior Art
Earth-orbiting spacecraft, such as communications satellites, require orientation control to minimize excessive movements in pitch, yaw, and roll, that can effect their remaining in proper orbit, and that can interfere with their pointing in a proper direction to insure the reception of signals transmitted therefrom at receiving stations on the ground. Various systems are provided on the spacecraft to affect this control involving momentum wheels, thrusters, magnetic torquers, and sensors for yaw, roll, and pitch. For example, one such control system uses momentum bias for preventing the set yaw orientation from drifting. However, the spacecraft is regularly undergoing disturbances, from such factors as solar torques and thruster firings, that can cause variations in the yaw angle from the desired orientation and beyond allowable ranges of operation. The current approach to dealing with this problem uses a combination of the yaw sensors and magnetic torquers to maintain the desired yaw orientation.
Unfortunately, there are variations in the earth's magnetic field and, indeed, it even disappears on occasion rendering the magnetic torquers less effective or even totally ineffective. Furthermore, there are often disturbances on the spacecraft itself which affect the capability of the magnetic torquers. For example, magnetic torquers are known to interact adversely with solar torques and with the B-Field, that is, the earth's magnetic field. Still another drawback of known systems is that when a maneuver is performed by the spacecraft, torque disturbances are thereby imparted to the spacecraft which are often sensed by the computer as a solar torque or a B-field anomaly. To avoid excessive correctional operations by the magnetic torquers under the direction of the on-board computer, it is therefore often necessary to temporarily disable the magnetic torquers, sometimes for an extended duration.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice. The actuation and deployment concept embodied by this invention is different from all of the devices reviewed above.